Fibre Optic Cable Characteristics

tyson -

Here are some takeaways from my recent Optic Fibre Endorsement Course ran by JB Hunter. You can read more about the course here.

Fibre Design

An optical fibre consists of a thin glass rod with two parts:

  • Core: The inner portion where light travels.
  • Cladding: The outer layer that reflects light back into the core, maintaining the light's path through total internal reflection.

Types of Fibre

Optical fibres are categorised into two main types based on light propagation:

  • Multi-mode Fibre: Features a large core allowing multiple light paths, sensitive to modal dispersion. It's easy to couple with light sources, cost-effective, and simpler to connect and splice. However, it has higher attenuation and lower bandwidth, limiting transmission to short distances (up to 2000 meters).
    • Step-Index Multi-mode Fibre: Uniform refractive index in the core, with core/cladding diameters of 50/125 or 62.5/125 microns.
    • Graded-Index Multi-mode Fibre: Non-uniform refractive index, decreasing from the centre outward, causing light to travel in a sinusoidal pattern.
  • Single-Mode Fibre: Smaller core, supports only one light mode, eliminating modal dispersion. It provides higher bandwidth and lower attenuation, suitable for long-distance transmission with core/cladding diameters of 9/125 microns.
Photo by Ildefonso Polo

Cable Types and Applications

  • Outside Plant (OSP) Cables: Used for outdoor applications, including:
    • Standard Loose Tube: Jelly-filled tubes stranded around a central strength member.
    • High Strength (HS) Loose Tube: Suitable for direct burial in harsh environments.
    • Ribbon Cables: Fully dielectric, designed for duct installation.
  • Indoor Cables: Used for indoor applications, including:
    • Tight Buffered Cables: Easier to connect and terminate, suitable for indoor runs.
    • Plenum and Riser Rated Cables: Designed to meet fire safety standards, used in buildings with higher fire risks.

Pre-terminated Fibre Cables

Pre-terminated cables come in fixed lengths or custom lengths with specified connectors, suitable for commercial, riser, and carrier applications.

Air Blown Fibre Cables

Constructed from empty tubes housed in a dense polyethylene jacket, these cables allow individual fibres to be "blown" into the tubes using compressed air, typically used in OSP situations.

Sheath Types for Distribution Areas

Different sheath types protect cables from environmental factors:

  • Loose Tube Cable: Used for small customer distributions and sometimes aerial applications.
  • Ribbon Cable: Protects against insect attacks.
  • Plenum and Riser Rated Cables: Used in high fire risk areas, emitting low smoke and no harmful substances.

Additional Sheath Protection

  • Armour Cables: Provide high fibre counts and protection against mechanical damage and corrosion.
  • Common Sheath Colours in Australia:
    • Green: National Broadband Network Company (NBN Co)
    • Blue: Telstra and other carriers like Pipe Networks and NextGen
    • Grey: Optus and AAPT for underground and aerial applications
    • Black: External applications, including aerial and underground ducts
    • Orange: Internal Multi-mode fibre cable
    • Aqua: Internal Multimode 50/125 – 850nm Laser-optimised
    • Yellow: Internal Single-mode fibre cable
    • Blue: Internal Polarisation Maintaining Single-mode

Mode Conditioning in Multi-mode Fibres

  • Importance: Mode conditioning is crucial for multi-mode fibres to handle varying input light sources. It addresses overfill or under-fill conditions.
  • Mandrel Wraps: Commonly misused for creating Equilibrium Modal Distribution (EMD). They actually strip off unstable edge modes, improving measurement consistency. Mandrel wraps should comply with TIA-568-C.0 for 50Ξm and 62.5Ξm core fibres.

Encircled Flux Compliance

  • Impact on Loss Testing: Beam geometry and modal distribution of the test source critically affect multi-mode fibre loss testing.
  • Historical Methods: Previously, long lengths of multi-mode fibre were used to standardise beam geometry, but this method may fail with modern fibres.
  • Standards Evolution:
    • Coupled Power Ratio (CPR): Defined in IEC 61300-1: Ed 2 (2004), but had limitations and industry confusion.
    • Encircled Flux (EF): Defined in IEC 61280-4-1 Ed 2 (2009), ensures symmetrical beam geometry and reduces loss test variability within 10%.

Carrier Performance Specifications

Telstra

  • Testing Wavelengths: Acceptance tests at 1550nm and 1625nm, commissioning tests at 1310nm and 1550nm.
  • Loss Budget Formulas:
    • Post 01/06: IL (1310nm) â‰Ī 0.35L + 0.1N + 0.3C + 0.3
    • Pre 01/06: IL (1310nm) â‰Ī 0.37L + 0.1N + 0.5C + 0.3
  • Max Attenuation:
    • Fibre: 0.35 dB/km @ 1310nm, 0.21 dB/km @ 1550nm (post 01/06)
    • Connectors: 0.30 dB (post 01/06), 0.50 dB (pre 01/06)
    • Fusion Splice: 0.10 dB

NBN Co

  • Optical Budget Constraints: Based on GPON standard ITU-T G.984 with a maximum optical loss of 28dB or distance of 15,000 meters.
  • Loss Components:
    • Fibre: 0.35 dB/km @ 1310nm, 0.21 dB/km @ 1550nm
    • Fusion Splice: 0.20 dB
    • SC/APC Connector: 0.30 dB
    • 1:32 Spliter: 16.7 dB
    • Multi-Fibre Connector: 0.62 dB

Customer Premises Standards

  • ISO/IEC 14763-3: 2014: Specifies maximum attenuation for connectors and splices.
    • Mated Ref to Ref Connection: MMF 0.10 dB @ 850 & 1300nm, SMF 0.20 dB @ 1310 & 1550nm
    • Non-Ref to Non-Ref: MMF 0.50 dB, SMF 0.75 dB
    • Splice: 0.30 dB
    • Cable: MMF 3.50 dB/km @ 850nm, SMF OS2 0.4 dB/km @ 1310 & 1550nm
  • Power Loss Budget: Total Connector Loss + Total Cable Loss + Total Splice Loss.